1. Field of the Invention
The present invention relates to a variable resonator, a tunable filter, and an electric circuit device using the same.
2. Description of the Related Art
In the field of high frequency radio communication, necessary signals and unnecessary signals are separated by taking out a signal of particular frequency from a large number of signals. A circuit serving the function is called a filter, and is mounted on many radio communication devices.
In a general filter, center frequency, bandwidth or the like representing the characteristics of the filter are invariant. To make radio communication devices using such a filter applicable for various frequencies, a method is easily considered in which a plurality of filters having different combinations of center frequencies and bandwidths are prepared and the filters are switched by a switch or the like corresponding to frequency application. In this method, filters are necessary by the number of desired combinations of center frequencies and bandwidths, and thus a circuit size increases. For this reason, the device increases in size. Further, it is impossible to operate the filter on frequency characteristics other than previously designed frequency characteristics that each filter has.
Patent literature 1 given below discloses a filter to solve the problem which has resonators using piezoelectric bodies. A bias voltage is applied to the piezoelectric bodies from outside to change the frequency characteristics (resonance frequency) of the piezoelectric bodies, and then the bandwidth of the filter is changed.    Patent literature 1: Japanese Patent Application Laid-Open No. 2004-007352
Non-Patent literature 1 given below discloses a resonator which has two microstrip line 802 arranged in a ring shape by allowing their end portions to face each other and whose facing end portions are connected by PIN diodes 10a (refer to FIG. 48). The center frequency of a filter is variable by using the resonator. Non-Patent literature 1: T. Scott Martin, Fuchen Wang and Kai Chang, “ELECTRONICALLY TUNABLE AND SWITCHABLE FILTERS USING MICROSTRIP RING RESONATOR CIRCUITS”, IEEE MTT-S Digest, 1988, pp. 803-806.
Although the tunable filter disclosed in the Patent literature 1 has a bandwidth as a ladder type filter, the changing width of the center frequency is as small as about 1% to 2% due to the limitation of the characteristics of the piezoelectric bodies. For this reason, variation of bandwidth is also about the same level, and a significant change of bandwidth is not possible.
By using the filter disclosed in the Non-Patent literature 1, the center frequency is variable but the bandwidth cannot be made significantly variable.